1. Field of the Invention
The present invention relates to an optical performance humidity dependency improving agent for cellulose acylate film. The invention also relates to a cellulose acylate film and a retardation film using the optical performance humidity dependency improving agent for cellulose acylate film, as well as to a polarizer and a liquid crystal display device using the cellulose acylate film or the retardation film.
2. Description of the Related Art
Use of liquid crystal display devices is expanding year by year as energy-saving and space-saving image display devices. Heretofore, one serious defect of liquid crystal display devices is that the display image viewing angle dependency of the devices is large. Recently, however, wide viewing angle liquid crystal display modes such as VA-mode and the like have become put into practical use, and accordingly, even in the market of televisions and others that require high-quality images, the demand for liquid crystal display devices is being rapidly expanding now.
The basic constitution of the liquid crystal display device comprises a liquid crystal cell with a polarizer arranged on both sides of the cell. The polarizer play a role of transmitting a light polarized in a predetermined direction alone, and the performance of a liquid crystal display device greatly depends on the performance of the polarizer therein. The polarizer generally comprises a polarizing element with a transparent protective film stuck to both sides thereof, in which the polarizing element is formed of a polyvinyl alcohol film or the like having adsorbed iodine or dye through alignment thereon. A cellulose acylate film of typically cellulose acetate has high transparency and can readily secure airtight adhesiveness to polyvinyl alcohol used as the polarizing element, and is widely used as a polarizer protective film.
It is known that disposing an optically biaxial retardation film between the polarizer and the liquid crystal cell in a liquid crystal display device realizes wider viewing angles, or that is, improves display characteristics. As the retardation film, a cellulose acylate film is specifically noted that can express excellent optical properties, concretely excellent in-plane retardation Re (nm) and thickness-direction retardation Rth (nm) of the retardation film; and such a cellulose acylate film is used in a liquid crystal display device as the retardation film therein.
On the other hand, cellulose acylate film has a problem in that, as compared with any other synthetic polymer, it readily absorbs water and therefore the film properties often change with the environmental humidity change therearound. With the recent tendency toward expanding use of liquid crystal display devices, large-size and high-definition use of televisions and others has become expanded, and the requirements for the quality of polarizer, retardation film and polarizer protective film are much increasing. In particular, large-size and high-quality liquid crystal display devices are desired to be used in various severer environments than before. From such viewpoints, the cellulose acylate film for use in liquid crystal display devices is earnestly desired to have improved resistance to humidity.
Regarding the above, a method has heretofore been investigated that comprises adding a hydrophobic compound to a cellulose acylate film as an optical performance humidity dependency improving agent for the cellulose acylate film to thereby prevent the film from absorbing water. As the optical performance humidity dependency improving agent for cellulose acylate film, mainly proposed are polyalcohol derivatives, polycarboxylic acid derivatives, compounds having a structure of both a polar group moiety and a hydrophobic group moiety, and ester-type plasticizers (see JP-A 2007-84692).
JP-A 2007-84692 discloses an example of adding an ester-type plasticizer, which has a benzenecarboxylic acid residue or a phenol residue at both ends thereof and has a structure containing an aliphatic cyclic glycol and an aliphatic cyclic dibasic acid, to a cellulose ester film. The patent reference says that, by adding the additive of the type thereto, a cellulose ester film can be provided which is excellent in resistance to humidity-dependent fluctuation of the optical properties thereof and does not physically degrade through reduction in the film thickness (thinning).
On the other hand, however, adding an additive that contains a nucleic acid base such as adenine, guanine or the like in a part of the skeleton thereof, to a cellulose acylate film has been heretofore investigated little from the viewpoint of improving the resistance to humidity of the film.
JP-A 2004-109410 discloses a retardation enhancer for cellulose ester, which comprises at least one compound having a combination of functional groups that express intermolecular interaction and capable of forming a molecular complex and contains a keto-enol tautomerizable compound. The patent reference discloses alanine and guanine as examples of the retardation enhancer for cellulose ester. However, JP-A 2004-109410 does not disclose an example where adenine or guanine is actually added to a cellulose acylate film to demonstrate the effect thereof, and, in fact, therefore, nothing has heretofore been disclosed or suggested at all relating to the humidity dependency-improving effect of the additive that contains a nucleic acid base such as adenine, guanine or the like in a part of the skeleton thereof, added to a cellulose acylate film.
The present inventors investigated the ester-type plasticizer described in JP-A2007-84692 in point of the optical performance humidity dependency-improving effect thereof for cellulose acylate film, and have found that the plasticizer is still ineffective and requires further improvement.